This patent application claims priority based on a Japanese patent application No. 2000-58810 filed on Mar. 3, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical member to be used in an apparatus using a light source of an ultraviolet laser having a wavelength of 400 nm or less, especially F2 excimer laser. The present invention particularly relates to a synthesized silica glass optical member suitable for a lens, a window member, a mirror, a prism, a filter, and the like, and a method for manufacturing it.
2. Description of the Related Art
Because synthesized silica glass contains a quite small amount of metal impurities that absorb light, and has high purity, the synthesized silica glass has high transparency over a wide wavelength range from a near-infrared region to a vacuum ultraviolet region to be transparent. In addition, the synthesized silica glass has a very small thermal expansion coefficient to be excellent in the stability of size. Accordingly, the synthesized silica glass has mainly been used as optical members of optical apparatus using the g-ray (436 nm) and the i-ray (365 nm) as their light sources conventionally.
Recently, as LSI""s (Large Scale Integrated circuits) have highly been integrated, a fine drawing technique capable of drawing lines narrower in width is required for an optical lithography technique for drawing integrated circuit patterns on wafers. Accordingly, the wavelengths of exposure light sources have been being shortened. As a result, KrF excimer lasers (248 nm) and ArF excimer lasers (193 nm), both having wavelengths shorter than the conventional g-ray (436 nm) and the i-ray (365 nm), have come to be used as light sources of steppers for lithography. Moreover, the examination about F2 excimer lasers (157 nm) having a shorter wavelength also has begun. On the other hand, photomask substrates for optical members to be used in the steppers are required to have characteristics such as optical transparency in the wavelength ranges to be used, stability and durability.
Conventional synthesized silica glass to be used in these optical systems produces new absorption bands in an ultraviolet region when it is irradiated by a high energy ray of, for example, a KrF excimer laser (248 nm) or an ArF excimer laser (193 nm) for a long time. The synthesized silica glass produces an absorption band of about 215 nm in wavelength, the so-called Exe2x80x2 center, and an absorption band of about 260 nm in wavelength, the so-called NBOHC (Non-Bridging Oxygen Hole Center). These absorption bands produce such undesirable phenomena in these optical systems as the decrease of their transmittancy, the increase of their absolute refractive indices, the changes of their distributions of the refractive indices, the generation of fluorescence, and the like. Consequently, the conventional synthesized silica glass has the aforesaid problems for being used as an optical member of optical systems having a light source of an excimer laser.
The mechanism of the generation of these absorption bands has not fully investigated, however the mechanism can be considered that the new absorption bands are produced owing to paramagnetic defects that are produced in the synthesized silica glass owing to the optical reactions that are expressed by the following formulae (1) and (2) and generated by the irradiation of a laser beam on such peculiar defects of the synthesized silica glass as an oxygen lacking type defect expressed by a chemical formula xe2x89xa1Sixe2x80x94Sixe2x89xa1and an oxygen excessive type defect expressed by a chemical formula xe2x89xa1Sixe2x80x94Oxe2x80x94Oxe2x80x94Sixe2x89xa1.
xe2x89xa1Sixe2x80x94Sixe2x89xa1+hxcexdxe2x86x922xe2x89xa1Si.xe2x80x83xe2x80x83(1)
xe2x89xa1Sixe2x80x94Oxe2x80x94Oxe2x80x94Sixe2x89xa1+hxcexdxe2x86x922xe2x89xa1Sixe2x80x94O.xe2x80x83xe2x80x83(2)
Accordingly, for the improvement of the transparency in the wavelength band of F2 excimer lasers, there have been made attempts to obtain the durability to the irradiation of the ultraviolet rays by making silica glass obtained by the vapor-phase axial deposition (VAD) method contain fluorine, or by making silica glass obtained by the flame hydrolysis method called as the direct method contain hydroxyl groups or hydrogen molecules.
For example, Japanese Laid-Open Patent Publication HEI 10-67521 discloses a method for adding a fluorine gas to a porous silica glass by making a raw material gas contain the fluorine gas at the time of forming the porous silica glass body by generating the flame hydrolysis of the raw material with a burner. Japanese Laid-Open Patent Publication HEI 11-240728 discloses a method for making a porous silica glass body transparent glass by heating the porous silica glass body in an atmosphere including a hydrogen gas and a water vapor at 1400xc2x0 C. or more. Moreover, Japanese Laid-Open Patent Publications HEI 8-67530 and HEI 8-75901 disclose a method of the heat treatment of a porous silica glass body in a hydrogen gas atmosphere after the vitrification of the porous silica glass body in an atmosphere of a SiF4/He gas.
However, even in the thus manufactured high purified synthesized silica glass, absorption bands owing to structural defects called as the Exe2x80x2 centers and the NBOHC""s appear to decrease the transmittancy thereof in an ultraviolet region extremely when the synthesized silica glass is irradiated by an excimer laser beam for a long time.
Therefore, it is an object of the present invention to provide a synthesized silica glass optical member and method therefor which overcome the above issues in the related art. This object is achieved by combinations described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the present invention.
Another object of the present invention is to improve the transmittancy of a synthesized silica glass, aforementioned, to which fluorine compound is added in the ultraviolet region and to suppress the generation of absorption and fluorescence emission by the irradiation of an excimer laser beam. Further object of the present invention is, therefore, to provide a synthesized silica glass optical member excellent in laser light durability characteristics as an optical glass member for transmitting ultraviolet rays, and to provide a method for manufacturing it.
According to a first aspect of the present invention, there is provided a method for manufacturing a synthesized silica glass optical member, said method comprising: providing a porous silica glass body; heating the porous silica glass body in an atmosphere containing hydrogen or oxygen, and sintering the porous silica glass body in an atmosphere containing fluorine compound.
According to a second aspect of the present invention, there is provided a synthesized silica glass optical member manufactured by the method above.
According to a third aspect of the present invention, there is provided a method for a lithography using a photo mask, in which the photo mask utilizes a glass optical member, said method comprising: providing a porous silica glass body for the glass optical member; heating the porous silica glass body in an atmosphere containing hydrogen or oxygen; and sintering the porous silica glass body in an atmosphere containing a fluorine compound.
This summary of the invention does not necessarily describe all necessary features of the present invention. The present invention may also be a sub-combination of the above described features. The above and other features and advantages of the present invention will become more apparent from the following description of embodiments taken in conjunction with the accompanying drawings.